Closed railway hopper cars with pneumatic systems for unloading them are well known and widely used for the transportation of powdered and granular products. For cars with positive pressure pneumatic systems, air is supplied from an external source to pressurize the interior of the car body and simultaneously fluidize the dry, bulk product carried within the car to enable it to be conveyed in a fluidized state by air flow through product discharge conduits from the car. An air pressure of about fifteen psi gage may be maintained within the hopper car during the unloading procedure. Ordinarily, the pneumatic discharge or unloading system associated with a pressure discharge railway hopper car includes an air supply conduit for directing a portion of the air supplied to the hopper car directly into the discharge conduit or line. The air pressure in the discharge line is generally maintained at two or three psi below the pressure within the hopper car.
Trinity Industries, Inc., the assignee of the present invention, manufactures and sells Power Flo.RTM. pressure discharge railway cars with pneumatic unloading systems. An example of aeration equipment and a pneumatic discharge system for removing dry, bulk material from hopper style containers is described and shown in U.S. Pat. No. 3,929,261 entitled Aeration Device and Method for Assisting Discharge of Material from Containers.
Flour, starch, and similar powdery food products are examples of dry, bulk material suited for loading, transportation and discharge with an enclosed hopper car having a positive pressure pneumatic unloading system. Any dry powder, granular, or pellet-type product may, usually to advantage, be transported in such hopper cars. An enclosed hopper car in cooperation with the pneumatic system protects the contents of the car and minimizes product losses during loading, transportation, and discharge processes. Also, pneumatic transfer is often the most cost effective, efficient method for handling large quantities of dry, bulk fluent materials.
Except for a spherical vessel, which is impractical and not necessary for the relatively low pressure involved, a generally cylindrical vessel is often the most efficient form for the body of a pressure discharge railway hopper car. The generally cylindrical wall of such a vessel is typically self-supporting with respect to forces due to internal pressure, which tend to produce uniform circumferential tensile stresses in portions of the cylindrical wall. A normal requirement for such containers is that the cylindrical wall be sufficiently thick to endure the tensile stress. A generally cylindrical shape is also effective in carrying its own load and the load of the product it contains, although supplementary longitudinally extending, vertical load-carrying members are often desirable, and usually necessary, to prevent buckling of the tank.
A disadvantage of a generally cylindrical car body for a pressure discharge railway hopper car is that it often does not effectively use the available AAR Plate "C" boundaries, which in end profile are essentially rectangular, except for small triangular cut-away areas in each corner. From the point of view of maximizing load carrying capacity, a car with an end profile approximating the AAR Plate "C" rectangle is desirable. Such a car would have flat sides and a flat roof, but it would require thicker wall plates and numerous side and roof stiffeners to keep the side and top walls from deforming, thus considerably increasing the costs of manufacturing parts and of assembling the car.